The invention relates to a material sample for carrying out material tests under biaxial stress conditions as well as to a material sample setup and a material sample testing machine in which the material sample finds use.
During the design and sizing of machine components and the calculation of the strength of the components, the typical material strength values of the materials are resorted to in order to keep the amount of material used in the component as small as possible and nonetheless to fulfill the requirements placed on the components.
Especially when expensive materials are used and in lightweight construction, there is still potential for further saving of material and/or reducing weight when the actual material properties are known precisely.
In many fields of technical application, materials are used not only in their pure form, but oftentimes the surfaces of the components are furnished with engineered surface coatings, which impart specific desired surface properties to the component, although they usually change the strength values, especially in the case of narrow or thin components. The processing of materials also exerts an influence on the strength values.
In addition, the strength values of homogeneous materials may also be direction-dependent.
In order to be able to truly exploit the potential saving lying in a precise dimensioning, it is thus necessary again and again to determine appropriately the strength values of materials in the individual case. A biaxial stress distribution, as also arises in reality, is used in order to take into account the directional dependence of the strength values in the testing method.
The prior art, for example, is the biaxial tensile test based on a so-called cross sample 100, as is shown in FIG. 2. A cross-shaped test piece 100 is produced from the material that is to be tested and is then placed under tension at the crossbars in an appropriate tensile testing machine. The desired characteristic values of the material are determined from the deformation/breakage of the sample. These samples are relatively complicated; conventional cross samples made of expensive material that is to be investigated have a span width of 270 mm with a thickness of 15 mm and weigh about 4 kg (titanium).
In order to generate a more uniform biaxial stress load in the material in departure from the classical tensile test, which is limited to two predetermined axes, centrifugal tests using model disks 200, as are shown in FIG. 3, are carried out at speeds of around 50,000 rpm. This enables the required strength determination of the disks to be obtained or else it is possible to determine the desired characteristic values of the material from changes in shape or breakage resulting from the forces arising in the disk. These test pieces are also complicated and expensive; designed disks have diameters of 220 mm and weigh about 3 kg (titanium). In addition, it is necessary here to include mounting steps in order, for example, to attach the disk to be tested to a hub, etc., before it can be tested.